Chaperonin—Co-chaperonin Interactions - The Networking of Chaperones by Co-chaperones - page 161

(Fenton and Horwich 2003 ; Frydman 2001 ; Hartl and Hayer-Hartl 2002 , 2009 ).
GroEL exhibits weak ATPase activity that is lowered in the presence of GroES
(Chandrasekhar et al. 1986 ; Goloubinoff et al. 1989b ). GroEL provides energy in the
form of ATP as the energy released during hydrolysis assists in the folding of non-
native proteins (Xu et al. 1997 ). Transformational changes in the trans ring caused
by binding of ATP, substrate and GroES to the cis ring results in the trans ring not
being able to bind substrate (Tyagi et al. 2009 ). ATP hydrolysis of the GroES-bound
ring is required for the binding of ATP to the trans ring, negative cooperativity is dis-
played between the two GroEL rings which favours dissociation of GroES, ADP and
substrate from the cis ring (Rye et al. 1997 ). If the substrate is not folded correctly it
can rebind to another or the same GroEL for successive cycles of folding (Rye et al.
1997 ). GroES can now bind to the trans ring and this ring then becomes the new cis
ring in the subsequent round of substrate folding events. Thus both rings alternate
to become the cis ring during folding cycles and this has led to the term “two-stroke
engine” for the GroEL/GroES folding machine (Lorimer 1996 ; Xu and Sigler 1998 ).
In addition to its role as a lid for the folding chamber in the chaperonin complex,
GroES controls the cooperativity by directing conformational changes in GroEL
that are orchestrated by the seven mobile loops binding to each of the seven GroEL
subunits, followed by release of substrate into the cage (Gray and Fersht 1991 ; Todd
et al. 1994 ; Yifrach and Horovitz 1995 ). GroES also plays a key role in controlling
the competence and specificity of protein folding by GroEL (Richardson et al. 2001 ).
Based on the GroEL-GroES-ADP complex, the binding of GroES causes large
rigid body movements of the apical domains of GroES that results in doubling of the
volume of the cis ring cavity compared to the trans ring (Fig. 8.2 ; Xu et al. 1997 ).
This increased volume is capable of binding a native protein of 70 kDa (Houry
et al. 1999 ). Most of the E. coli proteins that require GroEL-GroES for folding are
~ 60 kDa and larger proteins that cannot be accommodated within the folding cavity
can be folded by binding to the uncapped trans ring of GroEL (Sigler et al. 1998 ).
As a result of the limited number of GroEL-GroES dependent substrates, it has
been suggested that the complex may actively rescue proteins from kinetic folding
traps thereby facilitating their refolding (Hartl and Hayer-Hartl 2009 ; Jewett and
Shea 2010 ; Chakraborty et al. 2010 ). Without GroES and ATP, the most dependent
or stringent GroEL substrates do not fold and remain tightly associated with the
GroES as they need to transit through cis in order to fold (Rye et al. 1997 ). Binding
of GroES also causes a dramatic change in the walls of the cavity as the hydropho-
bic binding sites are rotated towards the interfaces of adjacent subunits and GroES
resulting in a hydrophilic wall; and the intermediate domain twists downwards cap-
ping the nucleotide binding site (Xu et al. 1997 ).
Roles of Bacterial Chaperonins
Due to their importance in protein homeostasis, chaperonins are essential and uni-
versally distributed in all bacteria. Bacterial chaperonins are required for the correct
assembly of the cell division apparatus (Ogino et al. 2004 ). In contrast to E. coli